though I still feel vanilla xcom makes it way too easy to create squads of psi super soldiers that can rival ethereals with enough effort put into them.
The reason for this is that X-Com soldiers can start with higher psi strength than ethereal commanders, and can train their psi skill higher as well. The vanilla ethereal commander has 65-75 psi strength and 50-58 psi skill. A psi skill of 50 is about what it takes to mind control an opponent with psi strength equal to your own. X-Com soldiers can be cheaply screened to all have 90+ psi strength and thus be able to mind control ethereal commanders with as little as 30 psi skill. Some soldiers come out of their first round of psi training already able to mind control ethereal commanders.
The way to nerf psionics for X-Com and require larger amounts of training to perfect it is to reduce maximum psi strength in soldiers. On the other hand, if you reduce maximum psi skill but leave psi strength maximum at 100, maximum psi power is reduced but the rapid training remains the same.
Another way to make it harder to mind control aliens without changing their own psi power is to raise their psi strength and lower their psi skill accordingly. For example: multiply their psi strength values by 1.5 and then divide their psi skill values by 1.5. The combined increase to psi strength and reduction to psi skill makes their psionic power exactly the same as before.
If you raise the minimum psi strength of X-Com soldiers, you reduce the amount of "sponge-brains" in the squad and thus reduce the aliens' success rate with psionics, especially when the psionic attacks are coming from weak aliens such as ethereal soldiers.
I've done some testing with different values. Here I'll give a number for soldier maximum psi strength and skill, and a description of how it plays out with vanilla aliens (assuming all soldiers in your squad have max or near max psi strength):
100/100: all aliens easy to mind control from the far end of the map; your soldiers basically immune to psi attacks
90/90: all aliens easy to mind control, but the most difficult ones you need not be too far away from; your soldiers nearly immune to psi attacks
80/80: all aliens can be mind controlled but the hardest ones may take several tries; your soldiers slightly vulnerable to psi attacks from ethereal commanders
70/70: all aliens can be mind controlled but the hardest ones have a very low success chance even up close; your soldiers vulnerable to ethereal commanders
60/60: most aliens can be controlled but training is very important, and psi attacks are pretty useless on only the first month of training; your soldiers slightly vulnerable to ethereal leaders
50/50: many aliens can be controlled but it takes high training, and minimizing range to target is important; your soldiers are vulnerable to ethereal leaders and slightly vulnerable to sectoid leaders
40/40: easy aliens can be controlled reliably at close range with max training; your soldiers are vulnerable to sectoid leaders and slightly vulnerable to ethereal soldiers
30/30: with perfect training, easy aliens can be controlled but not reliably; your soldiers are vulnerable to the weakest psionic attacks: from ethereal soldiers
20/20: some soldiers come out of training on the first month with max psi skill; few aliens can be controlled at all and none easily; all of your soldiers are sponge-brains
